In automatic screw fastening, a screw is inserted into a screw hole in a work, and a screwdriver fastens the screw. If a member that inserts the screw into the screw hole is the screwdriver itself, fastening can conveniently start just after the insertion. Accordingly, one conventionally proposed method accommodates a screw in a tray on a worktable, picks up the screw engaged with a bit utilizing magnetism and a screwdriver, inserts the screw into a screw hole in a work, and fastens the screw.
In addition, this method previously memorizes a position of a center axis of the screw or a position of a center axis of a hole for accommodating the screw, and moves the screwdriver so that the center axis of the bit can accord with the center axis of the screw.
Prior art include Patent Documents 1 and 2:    Patent Document 1: Japanese Laid-Open Patent Publication No. 3-221331.    Patent Document 2: Japanese Laid-Open Utility Model Publication No. 5-60736.
Even if the screwdriver's movement is controlled so that the center axis of screwdriver's bit can accord with the center axis of the recess of the screw, a positional shift occurs between both center axes on a plane orthogonal to both center axes in actually positioning the bit due to wear-out and changes with time of the motor and the transmission mechanism. When the bit is engaged with the recess while both center axes shift from each other, the bit rubs on the recess. As a consequence, the abrasion between the bit and the screw causes spread of the abrasion powder, and pollution and electric short circuit of the product. The deformation of the bit and the screw cause defective screw fastening with a predetermined torque, defective products, or an increased number of tool exchanges, hindering the highly efficient production.